Like many fish, the eel catfish is a typical suction feeder when it's in the water. It sucks in a mouthful of waterand prey.

But on land the eel catfish uses a completely different method. The creature lifts the front part of its body and bends its head down toward the ground.

This ability is essential to thriving on land, write the researchers whose findings will be published tomorrow in the journal Nature.

Evolution at Work?

The fish is significant for anyone studying evolution, says biology professor Dominique Adriaens of Ghent University in Belgium.

"It shows that a transition from aquatic feeding to terrestrial feeding is possible without the necessity of a lot of spectacular [body form] and functional changes and can thus be explained by Darwinian evolution," said Adriaens, who co-authored the new study.

But the eel catfish does show what Adriaens calls "a high degree of evolutionary plasticity [adaptability] in its total body plan."

The catfish has successfully adapted to life in swampy patches of forest terrain, the researchers say. Normally only much smaller fish thrive in these shallow waters.

What allows the eel catfish to flourish there is its elongated body and ability to feed on both land and water.

"Being a large fish in a habitat with the absence of large water bodiesat least for some periods of the yearputs some constraints on aquatic feeding," Adriaens said.

If a large fish is going to survive in such an environment, it needs another way to get food.

Necking for Food

Farish A. Jenkins, Jr., is a professor of zoology at Harvard University in Cambridge, Massachusetts, who was not involved in the study.

He calls the finding "a very interesting and valuable contribution in any number of respects."

First, Jenkins says, this species' behavior is a reminder that fish have climbed out of water many times. He cites the walking catfish, climbing perch, and mudskippers as other examples.

"As far as we now know, Tiktaalik roseae and related forms were simply the first to undertake the initial steps towards colonizing land," he said.

Jenkins also points out that significant changes in body form are required for terrestrial feeding.

Typically, he said, the mouth of a fish is at the end of its body, generally on a head that can't move independently of the rest of its body (as human heads can, thanks to our necks).

A typical fish's entire body can be moved in the three-dimensional aquatic environment, he explained. But this won't work on land.

"In the case of eel catfish, the eel-like shape of the body allows for the head to be flexed downwards on an elevated 'neck' region."

Chasing prey may not be the only reason a fish would want to get out of the water, Jenkins said. Other motivations might also encourage the move.

For example, if the oxygen supply is depleted or the water is drying up altogether, a fish would want to find another body of water. In situations like that, it helps to be able to slither along to the next wet spot.

The need to escape aquatic predators might also be a factor in determining whether a species evolves the ability to venture out on land.